The present invention relates generally to metal rolling mills and more particularly to a method of compensating for variations in workpiece gage due to irregularities in mill rolls.
In the discipline of metal rolling, it is a very common practice to employ what is commonly known as an automatic gage control (AGC) as a form of control of the workpiece thickness. In the well known form of gage control, the BISRA "gagemeter" system, (U.S. Pat. No. 2,680,978), a signal proportional to mill deflection is combined with a signal proportional to unloaded roll opening to form a signal proportional to thickness delivered from the rolling mill, which is used in a closed-loop control system to maintain desired workpiece thickness.
Such systems respond correctly to variations in strip characteristics, such as entry thickness and hardness, but incorrectly to variations in the roll gaging caused by roll irregularities.
As used in the present application, roll irregularities are basically of two kinds, both of which result from the imperfect grinding of the rolls or the supporting journals. The first of these irregularities is what is commonly referred to as eccentricity. Eccentricity is that irregularity which results from the roll having an improper center of rotation even though the exterior surface of the roll may be perfectly circular. The second irregularity is what is commonly referred to as ovalness and is that condition which exists when the roll is not of perfect circular configuration but does in fact have an oval cross-sectional configuration. Each of these irregularities produces a cyclic variation in the workpiece gage which is related to the roll rotational speed. In the case of eccentricity, the variation occurs once for each revolution of the roll. In the case of ovalness, the cyclic variation occurs twice for each revolution of the roll.
In a four-high mill, that is a mill having two backup rolls and two work rolls, although ovalness is sometimes a problem, the most common irregularity for the backup rolls is eccentricity resulting from improper centering of the roll journals during the grinding of the roll body. The work rolls in this particular situation are normally not supported at their ends but are in fact "free floating" and thus ovalness becomes the primary consideration. In a two-high stand in which only work rolls are present, the work rolls may exhibit both eccentricity and ovalness.
These problems of eccentricity and ovalness have been recognized and addressed in the prior art. For example, in U.S. Pat. No. 3,580,022, "Rolling Mill Including Gage Control" by M. D. Waltz et al. (issued May 25, 1971), there is provided a scheme for neutralizing the effects of eccentricity by essentially blocking out or masking the eccentricity force signal variations to prevent hunting by the AGC screw system. This system does not, however, compensate for the gage errors produced in the workpiece by such eccentricity.
Others have attempted by various means, such as templates patterned after the actual roll profile, to compensate for irregularities such as eccentricity and ovalness. Frequency analysis methods have also been proposed; for example, that described in U.S. Pat. No. 3,928,994 by K. Ichiryu et al., entitled "Thickness Control System for a Rolling Mill", issued Dec. 30, 1975, identifies the periodic components of output thickness variation from continuous measurement and statistical analysis of these variations for use in a gage control system. Like the method of Waltz, this method serves only to mask the periodic force components rather than compensate the roll irregularities. These systems, however, are expensive to implement and difficult to maintain since they normally require additional equipment physically coupled to the main support rolls which must be disconnected and reconnected each time these are changed and, in some methods, additional thickness gaging means.
A different approach has been used where it is expected that gage variations due to roll irregularities may exceed those due to entering thickness or hardness variations. In such cases, the roll gap control system may be designed to hold roll separating force constant. Where eccentricity frequency is much lower than the response limits of the roll gap control, this provides a very effective means of reducing gage variations due to roll irregularities, but it is totally ineffective in correcting for entering thickness or hardness variations.
A system which performs essentially as a constant gap or gagemeter control for strip related disturbances, but essentially as a constant force control for disturbances produced by the mill rolls would thus be ideal.